Method and apparatus for defrosting refrigeration systems



J. H. BLAKE METHOD AND APPARATUS FOR DEFROSTING Sept. 29, 1964 REFRIGERATIONS SYSTEMS Filed March 8, 1962 I5 Sheets-Sheet 1 H M-HM-H mus kkmmzmk 3G4 NN SOU mam c594 3:6 muaciumsuh :2: S

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mamuumsou INVENTOR. JULIAN H. BLAKE ATTOR NEYS Sept. 29, 1964 3,150,498

J- H. BLAKE METHOD AND APPARATUS FOR DEFROSTING REFRIGERATIONS SYSTEMS Filed March 8, 1962 S-Sheets-Sheet 2 Elia--2- INVENTOR.

JULIAN H. BLAKE ATTORNEYS Sept. 1964 J. H. BLAKE 3,150,498

ma'moo AND APPARATUS FOR DEFROSTING REFRIGERATIONS SYSTEMS Filed March 8. 1962 3 Sheets-Sheet 3 INVEN'A'OR. JULIAN .H. BLAKE BY W ATTORNEYS LIQUID SUB-COOLER United States Patent 3,150,498 METHOD AND APPARATUS FOR DEFROSTING REFRIGERATION SYSTEMS Julian H. Blake, Belmont, Califi, assignor to Ray Winther Company, South San Francisco, Calif., a corporation of California Filed Mar. 8, 1962, Ser. No. 178,408 19 Claims. (Cl. 62-81) This invention relates to improvements in a method and apparatus for defrosting refrigeration systems and more particularly to hot gas defrosting modes wherein the heat of compression of the refrigerant in its gaseous phase is utilized to effect rapid defrosting of the evaporators.

The great increase in the frozen food industry and the widespread tendency to centralize food stores into large supermarket operations has created a huge demand for multiple evaporator refrigeration systems. Economy of installation and operation makes it highly desirable to incorporate the evaporator coils of the frozen food counters, and other refrigeration requirements such as walk-in boxes and even air conditioning, in a single centralized refrigeration system.

Frozen foods must be maintained at a temperature of 0 F. or below and these low temperature requirements create considerable problems in connection with the frosting up of the evaporator coils. In order to prevent unwanted rise in temperature in the frozen food counter, defrosting of the evaporator coils must be performed as expeditiously as possible.

Developments have lagged in the art of defrosting multiple evaporators of the type set forth. In general, the methods used are the same as the methods originally conceived for use with small refrigerators and the like. These methods fall into two basic groups: (1) defrosting by the application of external heat such as the ambient heat of the atmosphere or heat from other external sources, and (2) the heat of compression produced during the refrigeration cycle and which is normally removed from the refrigerant by the condenser. It is the latter basic method of defrosting, commonly known as the hot gas defrost method, with which the present application is concerned.

Previous hot gas defrost systems have circulated the hot gas to the evaporator either by valving the system to cause the hot gas to cycle through the evaporator in a reverse direction or by isolating the evaporator in such manner as to permit a liquid vapor cycle wherein the liquid returns to the heating point under the influence of gravity. The latter system is much too slow for large scale operation and requires an excessive amount of plumbing.

The reverse cycling hot gas defrost systems present a major disadvantage in that at least a portion of the hot gas is usually condensed into liquid form in the evaporator coil during the defrosting operation. The liquid refrigerant is then drawn into the suction line of the compressor and passes therethrough in the form of liquid slugs. If these slugs reach the compressor in liquid form they can cause breakage and other damage. This is commonly called slugging the compressor.

To prevent damage to the compressor it has heretofore been necessary to distribute the liquid by revaporizing the slugs or by entraining the liquid in the stream of vaporized refrigerant in the form of small atomized particles. Either of these methods requires additional expensive apparatus such as heat means for effecting the revaporization or injection devices for atomizing the liquid slugs plus accumulating chambers which act somewhat in the manner of surge tanks. Not only is the added apparatus expen- "ice.

sive but under certain conditions slugging of the compressor is still possible.

It has been proposed to avoid the slugging problems by connecting a pair of evaporators in series with the first evaporator operating at subfreezing temperatures and the second evaporator operating at above freezing temperatures. With this system only the subfreezing evaporator needs defrosting and it is possible to cycle the hot gas from the compressor through the subfreezing evaporator, to effect defrosting thereof, and into the above-freezing evaporator where any liquefied refrigerant will be expanded into gaseous form. This is really only an adaptation of the revaporizing technique wherein the revaporizing heat is acquired from the air passing over the abovefreezing evaporator coil. While this system is somewhat more eflicient than revaporizing by the addition of external heat or by utilizing the heat of compression, it is rather tricky in operation and will function only where high and low temperature coils are connected in series.

The present invention proposes a hot gas defrost system wherein the refrigerant liquefied during the defrosting process is utilized for refrigerating without revaporizing or passing through the compressor, thus adding to the thermal efiiciency of the system and obviating any requirement for separate vaporizing or atomizing means.

Accordingly it is a principal object of this invention to provide an improved refrigeration system wherein multiple evaporator coils may be selectively defrosted by reverse cycling of the compressed hot gaseous refrigerant and wherein the cooled and partially liquefied refrigerant will be returned to the liquid refrigerant supply line for use in refrigerating the other evaporators in the system.

Another object of the present invention is to provide a system of the character described in which the relative pressures of the hot gas supply and the liquid refrigerant supply are regulated to provide a pressure difierential which will cause the cooled refrigerant to be injected into the liquid refrigerant supply.

A further object of the invention is to provide an apparatus of the character described in which the pressure in the hot gas supply line is maintained at a constant value and the pressure in the liquid supply line is reduced to a value below that of the hot gas supply line so as to create said pressure differential.

A still further object of the invention is to provide a system of the character described in which the liquid refrigerant is subcooled prior to reduction in pressure so as to prevent flash vaporization thereof.

Another object of the invention is to provide a method of defrosting an evaporator coil by supplying hot vaporized refrigerant to the suction line end of the evaporator and controlling the pressure in the liquid refrigerant supply line so as to create a pressure differential across the evaporator unit sufficient to urge refrigerant condensed therein into the liquid refrigerant supply system.

Still another object of the invention is to provide a method of defrosting an evaporator coil of the character described in which the pressure at which the hot vaporized refrigerant is supplied to the evaporator is less than the output pressure of the compressor and in which the pressure of the liquid in the refrigerant supply is less than the pressure in the hot vaporized refrigerant supply but which is still high enough to maintain the refrigerant supply in liquid form for expansion in additional evaporator units.

Another object of the invention is to provide a method and apparatus for accomplishing hot gas defrosting of a plurality of evaporators connected in parallel between a liquid refrigerant supply line and a compressor suction line in which the individual evaporators may be selectively disconnected from the compressor suction line and connected to a hot gas supply line having a pressure higher than the pressure in the liquid refrigerant supply line.

Yet another object of the invention is to provide a large-scale multiple evaporator refrigeration system capable of accomplishing rapid hot gas defrosting of the individual evaporators on demand and with a minimum of controls and plumbing.

Another object of the invention is to provide a refrigeration system of the character described in which slugging of the compressor is completely avoided.

Further objects and advantages of my invention will be apparent as the specification progresses, and the new and useful features of my method and apparatus for defrosting refrigeration systems will be fully defined in the claims attached hereto.

In general, the present invention provides a method and apparatus for defrosting a refrigeration system by passing hot vaporized refrigerant through the evaporator under the influence of a pressure differential sufiicient to cause cooled and partially liquefied refrigerant to flow into the liquid refrigerant supply line. The method and apparatus are particularly suited for effecting hot gas defrosting of multiple evaporators connected in parallel, the apparatus having valves for controlling the pressures in the hot gas supply line and in the liquid refrigerant supply line together with valve means for selectively connecting each of the evaporators to either the compressor suction line, for refrigerating action, or to the hot gas defrost line, for defrosting action. During the defrost action the cooled and partially liquefied refrigerant is injected into the liquid refrigerant supply line by reason of the described pressure differential, thus making the liquefied refrigerant available for refrigerating action in others of the multiple evaporators and at the same time avoiding problems of slugging the compressor.

The preferred forms of my invention are illustrated in the accompanying drawings forming part of this specification in which:

FIGURE 1 is a schematic plan view illustrating a preferred embodiment of the refrigeration system of the present invention, particularly adapted for use in supermarkets or the like;

FIGURE 2, an enlarged view of certain of the components of FIGURE 1 shown in operative association; and

FIGURE 3, a schematic plan view illustrating a refrigeration system constructed in accordance with the present invention and adapted for use with a single evaporator.

While I have shown only the preferred forms of my invention, it should be understood that various changes or modifications may be made within the scope of the claims attached hereto without departing from the spirit of the invention.

Referring to the drawings in greater detail, it is seen that the defrosting apparatus for refrigeration systems of the present invention is particularly adapted for use in a system having an evaporator unit 11 connected between a liquid refrigerant supply line 12 and a compressor suction line 13 together with valve means 14 for selectively connecting a hot gas supply line 16 to the evaporator unit 1 1 while at the same time disconnecting the latter from the compressor suction line 13, the system also including means 17 for regulating the pressure in the liquid refrigerant supply line 12 so as to create a pressure differential across the evaporator unit 11 sufiicient to urge refrigerant condensed therein into the liquid refrigerant supply line when the hot gas supply line 16 is connected to the evaporator unit.

The method and apparatus of the present invention is particularly suited for use in connection with conventional refrigeration systems in which compressor means 18 compresses a gaseous refrigerant and propels it through a conduit 19 into a condensing means 21 adapted to remove a portion of the heat energy from the refrigerant and condense it into liquid form. The liquid refrigerant then passes through a conduit 22 into a receiver 23 and from the receiver into the liquid refrigerant supply line 12 for the evaporator unit 11.

The evaporator unit 11 includes an expansion valve 24 adapted for expanding the liquid refrigerant in the evaporator coil 26, this action serving to absorb heat energy and reduce the temperature of the coil 26 for refrigeration purposes.

In accordance with the present invention, a predetermined pressure differential is maintained between the pressure in the hot gas defrost line, leading from the compressor discharge conduit 19 and the liquid refrigerant supply line 12, the pressure differential being sufiicient to urge the cooled and at least partially liquefied refrigerant from defrosting evaporator coil 26 into the liquid refrigerant supply line 12 where it will be available for refrigerating the evaporator units not being defrosted. As here shown, the valve means 14 is adapted for selectively connecting the end 27 of the evaporator coil remote from the expansion valve 24 to the compressor suction line 13 during the refrigeration cycle or to the hot gas defrost line 16 during the defrost cycle. When the evaporator unit 11 is in the defrost cycle, the cooled and partially liquefied refrigerant is diverted around the expansion valve 24 by means of a by-pass 28 containing a one-way check valve 29.

As here shown, the means 17 for controlling the pressure in the liquid refrigerant supply line 12 so as to achieve the desired pressure differential between lines 12 and 16 comprises a conventional pressure-regulator valve 31 interposed in the line 12 between the receiver 23 and the first evaporator unit 11. The pressure-regulator valve 31 may be of any suitable type and it is preferred to use a conventional pressure-reducing valve of the type well known in the art.

The valve 31 is adjusted to provide for a pressure in line 12 which is lower than the pressure in the hot gas defrost line 16. The pressure differential should be sulficient to cause the refrigerant to move rapidly through the evaporator coil 26 in the direction opposite to its normal direction of flow, the refrigerant passing through by-pass 28 and check valve 29 and arriving at liquid refrigerant supply line 12 at a pressure higher than the pressure maintained in line 12 by the pressure-regulator valve 31. The cooled refrigerant will thus enter the line 12 and supplement the supply of liquid refrigerant therein for subsequent use in the refrigerating cycles of other evaporator units communicating with line 12, or of the evaporator unit being defrosted, after the defrost cycle is completed.

In order to ensure that the pressure differential between lines 12 and 16 is of the required magnitude at all times, I prefer to mount a second pressure-regulator valve 32 in the hot gas defrost line 16 between the compressor discharge conduit 19 and the first of the evaporator units 11. The regulator valve 32 is set to provide a pressure in line 16 which is higher than the pressure in line 12 but which is slightly lower than the normal discharge pressure of the compressor means 18. For example, in a typical installation where the discharge pressure of the compressor means is 180 p.s.i., the valve 32 may be set to provide a pressure in the hot gas defrost supply line of 175 p.s.i. while the valve 31 is regulated to provide a pressure of about p.s.i. in the liquid refrigerant supply line 12, thus providing a pressure differential of about 15 p.s.i. The exact pressure differential to be used will depend upon the flow characteristics through the evaporator units but I have found in actual practice that a pressure differential of about 10 p.s.i. to 20 p.s.i. will be sufficient in most installations.

Preferably, the condensing pressure in the condenser means 21 is regulated to provide a minimum pressure in conduit 19 at least 5 pounds higher than the setting on the hot gas supply regulator valve 32 so as to ensure that the pressure in the hot gas supply line will not drop below the preselected value. Any of the conventional methods for controlling minimum condensing pressure, such as regulating water flow to water-type condensers, pressureregulator valves in conduit 19 between line 16 and the condenser means, or other equivalent methods, may be employed.

As previously stated, the method and apparatus of the present invention is particularly adapted for use with multiple evaporator installations such as those encountcred in large food markets wherein a plurality of evaporator units 11A, 11B and 11C are provided for maintaining subzero temperatures in frozen food cabinets and in which additional higher temperature evaporator units 11D are employed for other purposes such as walk-in refrigerators, etc. A system of this character is depicted schematically in FIGURES l and 2 of the drawings. As there shown, each of the evaporator units 11 comprises three evaporator coils 26. It should be understood, however, that the evaporator coils in the particular unit may be varied in number and arrangement so long as the described connections are provided.

It is necessary to the purposes of the present invention that the evaporator units 11 be connected in parallel between the liquid refrigerant supply line 12 and the compressor suction line 13 so that the refrigerant injected into supply line 12 from the evaporator unit 1113 being defrosted may be utilized in the refrigeration cycles of the other evaporator units 11A and 11C. I have found it desirable for at least two of the evaporator units to be in their refrigeration cycles for each evaporator unit being defrosted. In order to accomplish such action, the means 14 may be provided in the form of a two-position valve 33 having an electrically operated actuating device 34 supplied with electrical energy through leads 36. The valves 33 may be controlled by manual switches or by automatic defrost switching means having provision for ensuring that no more than the desired number of evaporator units are being defrosted at once.

As a feature of the present invention, subcooler means 37 is provided for maintaining the liquid refrigerant in the supply line 12 at a temperature sufficiently low to prevent flash vaporization as the liquid refrigerant passes from the receiver 23 and dryer 38 through the pressure-regulator valve 31. The subcooling of the liquid refrigerant also tends to cool the refrigerant being injected into line 12 from the evaporator unit undergoing the defrost cycle sufficiently to ensure that any vaporized refrigerant not condensed in the evaporator unit will be condensed to liquid form as it enters the supply line 12.

As here shown, the subcooler means includes a subcooler evaporator coil 39 surrounding the liquid refrigerant supply line 12 and having an expansion valve 41 interposed in a conduit 42 connected to the liquid refrigerant supply line 12. An opposite end of coil 39 is connected by means of conduit 43 to the compressor suction line 13.

As may be seen in FIGURE 1 of the drawings for multiple evaporator coil systems, I prefer to provide a two-stage compressor means 18. This means includes a low-stage booster compressor 44 interposed in the suction line 13 and discharging into a high-stage compressor 46 which discharges into the conduit 19. The conduit 43 is connected to line 45 between the compressors 44 and 46 in the conventional manner for desuperheating the gas entering compressor 46. The subcooler evaporator valve 41 may conveniently be operated by a sensing device 47 positioned at the intake side of compressor 46 in the conventional manner.

In operation, the valve means 14 will be initially set at the position illustrated in connection with the evaporator units 11A and 11C, that is, connecting the evaporator coils 26 to the compressor suction line 13. With the compressors operating, liquefied refrigerant maintained at a pressure of say 160 p.s.i. by valve 31, will flow through the expansion valves 24 and the expansion of the refrigerant will cool the evaporator coils as it assumes its vapor phase. When frost has accumulated on the evaporator coils 26, the valve means 14 for the particular evaporator will be displaced to its other position (illustrated in connection with evaporator unit 11B) communicating the evaporator coils with the hot gas supply line 16.

With the pressure in line 16 maintained at say p.s.i. by valve 32, the hot gas will flow in a reverse direction through the evaporator coils 26, the -by-pass 28 and the check valve 29 into the liquid refrigerant supply line 12. Should a more rapid defrosting action be desired, the pressure-regulating valves 31 and 32 could be adjusted to provide a greater pressure differential between lines 12 and 16. Conversely, if it is desired to slow down the defrosting action, the valves 31 and 32 could be adjusted to provide a lesser pressure differential. For most purposes, a differential of approximately 15 p.s.i. will be preferred.

As the hot vaporized refrigerant passing through coils 26 gives up its heat energy to effect defrosting, it becomes wholly or partially condensed and the cooled refrigerant passes through by-pass 28 and check valve 29 into the liquid refrigerant supply line 12. This cooled refrigerant adds to and supplements the liquid refrigerant flowing through the line 12 and is used during the refrigeration cycles of the other evaporator units 11A, 11C and 11D. In this connection it should be noted that the evaporator units 11D as depicted herein are intended to represent coils which are maintained at a temperature above the freezing point of water and consequently no defrosting of these units would be required.

It is highly desirable that at least two evaporator units be in refrigeration condition for each evaporator unit which is in defrost condition in order to provide a supply of gas to the compressor which is adequate for rapid defrosting. If desired, one or more of the evaporator units in the refrigeration cycle can be blocked open while another unit is being defrosted so as to permit constant operation of the compressor and consequent continuous defrosting action.

FIGURE 3 of the drawings illustrates a refrigeration system wherein a single evaporator unit is defrosted in accordance with the method and apparatus of the present invention. As there shown, the evaporator units 51 are connected in parallel between a liquid refrigerant supply line 52 and a compressor suction line 53. Two-position valve means 54 is provided at one end of each evaporator unit 51 for selectively connecting the latter to the compressor suction line 53 or to a hot gas supply line 55. A compressor 58 is provided with a discharge conduit 59 communicating with a condenser means 61 which has a discharge conduit 52 leading into a receiver 63, the receiver in turn being connected to the liquid refrigerant supply line 52.

The system as described so far is essentially similar to the system illustrated in FIGURES l and 2 of the drawings. The system of FIGURE 3 functions in a similar manner to the system of FIGURE 1, a differential in pressure being maintained between the hot gas supply line 55 and the liquid refrigerant supply line 52 which is sufficient to cause the partially liquefied refrigerant to pass from the evaporator unit into the liquid refrigerant supply line 52 during the defrost cycle, that is when valve 54 has been turned to th position shown in the drawing so as to communicate line 56 with line 52 through the evaporator unit.

In the form of the invention shown in FIGURE 1, the partially liquefied refrigerant passing from the evaporator unit during defrosting thereof is merely added to the liquid refrigerant supply flowing through line 12 and is immediately used by one or more of the other evaporator units which are in their refrigeration cycle. The form of the invention illustrated in FIGURE 3, on the other hand, injects the partially condensed refrigerant from the evaporator unit into the liquid refrigerant supply line 52 in such manner that a portion thereof may be stored for 3,1 50,498 7 E subsequent use in cooling the evaporator units. The units in a cycle where'the units appear in the order given,

liquefied refrigerant being substantially incompressible, it is necessary to modify the system slightly in order to provide storage space for the added liquid refrigerant while still maintaining the required pressure differential. In this connection, I have found it practicable to utilize the storage capacity of the receiver 63 by mounting the pressure-reducing valve means 57 (corresponding to means 17 of FIGURE 1) in line 62 between the condenser 61 and receiver 63. The refrigerant passing into line 52 from the evaporator 51 during the defrost cycle will be stored in receiver 63 and will be added to the supply of liquid refrigerant available to the evaporator expansion valves 66.

From the foregoing it will be seen that I have provided a novel method and apparatus for defrosting a refrigeration system in which the thermal efficiency of the system is increased while completely eliminating any necessity of separate apparatus for revaporizing or atomizing the cooled defrost refrigerant, since such refrigerant is not returned to the compressor directly but rather is added to the liquid refrigerant supply. In addition I have provided a novel means for controlling the pressure differential between the hot gas supply and the liquid refrigerant supply line so as to afford the described mode of operation of the invention.

I claim:

1. The method of defrosting evaporator units connected in parallel at one end to a liquid refrigerant supply line and selectively connectable at their opposite ends to a compressor suction line and a hot gas supply line, which comprises the steps of connecting the evaporator unit to be defrosted to the hot gas supply line, connecting the other evaporator units to the compressor suction line, controlling the pressure in the hot gas supply line to a predetermined value, supplying liquid refrigerant in the liquid supply line at a temperature below the condensation temperature, and controlling the pressure in the liquid refrigerant supply line to a lesser value than the pressure in the hot gas supply line but at a pressure high enough to retain substantially all of the liquid refrigerant in the liquid state in the refrigerant supply line, said pressure also being low enough to create a pressure differential across the evaporator unit being defrosted sufficient to urge refrigerant condensed therein into said liquid refrigerant supply line for passage through the evaporator units not being defrosted.

2. The method as described in claim 1, wherein the pressure in said liquid refrigerant supply line is reduced below the pressure in said hot gas supply line so as to provide a pressure differential therebetween of from about to about pounds per square inch.

3. The method of defrosting a refrigeration system having a compressor, a condenser, a receiver, a liquid refrigerant supply line, a plurality of evaporator units connected in parallel to the refrigerant supply line, a compressor suction line, and conduits connecting the units in a cycle where the units appear in the order given, comprising the steps of taking compressed hot refrigerant gas from the compressor and reducing its pressure to provide a controlled source of hot gas under pressure, reducing the pressure of the liquid refrigerant in the liquid refrigerant supply line to a pressure from 10 to 25 p.s.i. lower than said reduced pressure of the hot gas, directing hot gas at the controlled reduced pressure into one of the evaporator units and through the same into the liquid supply line to effect defrosting of said unit while supplying the remainder of the evaporator units with liquid refrigerant from the compressor suction line for normal refrigerating operation.

4. The method of defrosting a refrigeration system having a compressor, a condenser, a receiver, a liquid refrigerant supply line, a plurality of evaporator units connected in parallel to the refrigerant supply line, a compressor suction line, and conduits connecting the comprising the steps of taking compressed hot refrigerant gas from the compressor and reducing its pressure to provide a controlled source of hot gas under pressure, reducing the pressure of the liquid refrigerant in the liquid refrigerant supply line to a pressure from 10 to 25 p.s.i. lower than said reduced pressure of the hot gas, cooling the liquid in the liquid refrigerant supply line by removing heat energy therefrom prior to reducing the pressure thereof an amount sufficient to prevent flash vaporization of refrigerant in the liquid refrigerant Supply line as the pressure is reduced, and directing hot gas at the controlled reduced pressure into one of the evaporator units and through the same into the liquid supply line to effect defrosting of said unit while supplying the remainder of the evaporator units with liquid refrigerant from the receiver for normal refrigerating operation.

5. The method of defrosting a refrigeration system as defined in claim 4 in which at least two evaporator units are operating on a refrigeration cycle for each evaporator unit operating on a defrost cycle.

6. The method of defrosting a refrigeration system as defined in claim 4, in which the condensing pressure at the condenser is maintained at a pressure at least 5 psi. higher than the adjusted pressure of the hot gas.

7. The method of defrosting a refrigeration system having a compressor, a condenser, a receiver, a liquid refrigerant supply line, a plurality of evaporator units connected in parallel to the refrigerant supply line, a compressor suction line, and conduits connecting the units in a cycle where the units appear in the order given, comprising the steps of taking hot gas from the compressor and maintaining same at a constant pressure of about p.s.i., reducing the pressure in the liquid refrigerant supply line to a constant pressure of about 160 p.s.i., cooling the liquid in the liquid refrigerant supply line by removing heat energy therefrom prior to reducing the pressure thereof an amount sufficient to prevent flash vaporization of refrigerant in the liquid refrigerant supply line as the pressure is reduced, maintaining the condensing pressure in the condenser at a pressure above about p.s.i., passing said hot gas through one of the evaporator units to defrost the coils and condense the hot gas therein, and adding the condensed hot gas to the liquid refrigerant supply line Where it is effective to provide cooling in the remaining evaporator units.

8. In a refrigeration system, a liquid refrigerant supply line, a compressor suction line, a plurality of evaporator units connected in parallel between said liquid refrigerant supply and compressor suction lines, a hot gas supply line, means for selectively connecting said hot gas supply line to said evaporator units and at the same time disconnecting the latter from said compressor suction line, and means for regulating relative pressures in said liquid refrigerant supply line and said hot gas supply line comprising a pressure regulating valve mounted in said hot gas supply line upstream of said evaporator units and formed for reducing the pressure in such line to a constant predetermined value, and a pressure regulating valve mounted in said liquid refrigerant supply line upstream of said evaporator units and formed for reducing pressure in said line to a value below the pressure in said hot gas supply line downstream of said first-named pressure regulating valve for creating a pressure differential across the evaporator unit when said evaporator unit is being defrosted.

9. A refrigeration system as described in claim 8 which which also comprises subcooler means in said liquid refrigerant supply line upstream of said pressure-regulator valve formed for maintaining the liquid refrigerant in said line at a temperature sufficiently low to prevent flash vaporization as the liquid refrigerant passes through said pressure-regulator valve.

10. A refrigeration system as described in claim 9 wherein said subcooler means includes a subcooler evaporator coil in heat exchanging relation to said liquid refrigerant supply line and connected to said compressor suction line, and expansion valve means connected between said subcooler evaporator coil and said liquid refrigerant supply line for diverting a portion of the liquid refrigerant therefrom so as to effect subcooling of the rest of said liquid refrigerant passing through said liquid refrigerant supply line.

11.- A refrigeration system comprising a compressor, a. condenser receiving refrigerant from the compressor, a receiver receiving refrigerant from the condenser, a liquid refrigerant supply line receiving the refrigerant from the: receiver, a plurality of units of evaporator coils, expansion valve means connected between each unit of the evaporator coils and the liquid refrigerant supply line, a one-way by-pass valve around each of said expansion valve means, a suction line carrying refrigerant from the evaporator units to the compressor, a hot gas line carrying hot refrigerant from the compressor to the evaporator! units, a two-position valve between each unit of the evaporator coils and the suction line adapted to shut off communication to the suction line and to connect the unit with the hot gas line, a pressure-regulator valve in said liquid refrigerant supply line formed for maintaining a substantially constant pressure therein lower than the pressure in said hot gas line and a pressure-regulator valve mounted in said hot gas line for maintaining substantially constant pressure therein higher than the pressure maintained by the pressure-regulator valve in the liquid refrigerant supply line whereby hot gas will flow into each of the evaporator coils from the hot gas line when the two-position valve for such coil is set to communicate the evaporator coil with the hot gas line, said gas condensing in said evaporator coils while achieving a defrosting operation and passing through the one-way bypass valve into the liquid refrigerant supply line for reuse in other evaporator coils as a refrigerant.

12. A refrigeration system as described in claim 11 wherein control means is provided for said two-position valves whereby at least two evaporator coils will be connected in the refrigeration cycle for each evaporator coil connected in the defrost cycle.

13. A refrigeration system as described in claim it wherein subcooler means is mounted in said liquid refrigerant supply line between said receiver and said evaporator coils, said subcooler means being formed for maintaining the liquid refrigerant in said line at a temperature sufiiciently low to prevent fiash vaporiz'ation as the liquid refrigerant passes through said pressureregulator valve.

14. A refrigeration system as described in claim 13 wherein said subcooler means includes a subcooler evaporator coil surrounding said liquidrefrigerant supply line and connected to the suction side of said compressor, and expansion valve means connected between said sub cooler evaporator coil and said liquid refrigerant supply line for diverting a portion of the liquid refrigerant therefrom so as to effect subcooling of the rest of said liquid refrigerant passing through said liquid refrigerant supply line.

15. A refrigeration system as described in claim 14, wherein there are two compressors, one of which is a lowstage booster compressor receiving refrigerant from said two-position valves of said evaporator coils while maintaining a controlled pressure thereat and delivering such refrigerant in partially compressed form to the other compressor, the latter compressor comprising a high-stage compressor formed to further compress the refrigerant for delivery to said condenser, said hot gas line being connected to the discharge side of said last named compressor, and said subcooler evaporator coil being connected to theintake side of said last named compressor.

16. A refrigeration system as described in claim 15 wherein at least one group of air-cooling relatively high- 10 temperature evaporator coils operating at temperatures above the freezing point of water are connected between the liquid refrigerant supply line and the intake side of the high-stage compressor.

17. A refrigeration system comprising a compressor, a condenser connected to the discharge side of said compressor, a receiver connected to said condenser, a plurality of evaporator units, expansion valves at one end of said evaporator units connected to said receiver, means for selectively connecting the opposite ends of said evaporator units to the intake and discharge of said compressor, one-way check valves connected in by-pass relation around said expansion valves, and a pressure-regulating valve interposed in the connection between said receiver and said condenser and formed for reducing the pressure in said receiver to a value lower than the discharge pressure of said compressor so as to create a desired pressure drop across said evaporator units when connected thereto.

18. In a refrigeration system, a liquid refrigerant supply line, a compressor suction line, a plurality of evaporator units connected in parallel between said liquid refrigerant supply and compressor suction lines, a hot gas supply line, means for selectively connecting said hot gas supply line to one of said evaporator units and at the same time disconnecting the latter from said compressor suction line, a pressure-regulator valve in said liquid refrigerant supply line upstream of said evaporator units for regulating the pressure in said liquid refrigerant supply line to create a pressure differential across said evaporator units sufficient to urge refrigerant cooled therein into said liquid refrigerant supply line when said hot gas supply is connected to said evaporator unit, and sub-cooler means in said liquid refrigerant supply line upstream of said pressure-regulator valve formed for maintaining a liquid refrigerant in said line substantially in a liquid state by removing heat therefrom and cooling to a temperature sufiiciently low to prevent flash vaporization as the liquid refrigerant passes through said pressure-regulator valve.

19. A refrigeration system comprising a compressor, a condenser receiving refrigerant from the compressor, a receiver receiving refrigerant from the condenser, a liquid refrigerant supply line receiving the refrigerant from the receiver, a plurality of units of evaporator coils, expansion valve means connected between each unit of the evaporator coils and the liquid refrigerant supply line, a oneway by-pass valve around each of said expansion valve means a suction line carrying refrigerant from the evaporator units to the compressor, a hot gas line carrying hot refrigerant from the compressor to the evaporator units, a two-position valve between each unit of the evaporator coils and the suction line adapted to shut off communication to the suction line and to connect the unit with the hot gas line, a pressure-regulator valve in said liquid refrigerant supply line formed for maintaining a substantially constant pressure therein lower than the pressure in said hot gas line, and means for removing heat energy from the liquid refrigerant to prevent flash vaporization as the liquid refrigerant passes through said pressure regulator valve and to maintain substantially all of the liquid refrigerant in the liquid supply line in a liquid state.

References Cited in the file of this patent UNITED STATES PATENTS 1,601,445 Hilger Sept. 28, 1926 2,133,521 Wussow et a1 Oct. 18, 1938 2,141,715 Hilger Dec. 27, 1938 2,496,143 Backstrom Jan. 31, 1950 2,503,212 Patterson Apr. 4, 1950 2,553,623 Zumbro May 22, 1951 2,841,962 Richards July 8, 1958 2,874,550 Musson Feb. 24, 1959 2,966,043 Ross Dec. 27, 1960 2,978,877 Long Apr. 11, 1961 

1. THE METHOD OF DEFROSTING EVAPORATOR UNITS CONNECTED IN PARALLEL AT ONE END TO A LIQUID REFRIGERANT SUPPLY LINE AND SELECTIVELY CONNECTABLE AT THEIR OPPOSITE ENDS TO A COMPRESSOR SUCTION LINE AND A HOT GAS SUPPLY LINE, WHICH COMPRISES THE STEPS OF CONNECTING THE EVAPORATOR UNIT TO BE DEFROSTED TO THE HOT GAS SUPPLY LINE, CONNECTING THE OTHER EVAPORATOR UNITS TO THE COMPRESSOR SUCTION LINE, CONTROLLING THE PRESSURE IN THE HOT GAS SUPPLY LINE TO A PREDETERMINED VALUE, SUPPLYING LIQUID REFRIGERANT IN THE LIQUID SUPPLY LINE AT A TEMPERATURE BELOW THE CONDENSATION TEMPERATURE, AND CONTROLLING THE PRESSURE IN THE LIQUID REFRIGERANT SUPPLY LINE TO A LESSER VALUE THAN THE PRESSURE IN THE HOT GAS SUPPLY LINE BUT A PRESSURE HIGH ENOUGH TO RETAIN SUBSTANTIALLY ALL OF THE LIQUID REFRIGERANT IN THE LIQUID STATE IN THE REFRIGERANT SUPPLY LINE, SAID PRESURE ALSO BEING LOW ENOUGH TO CREATE A PRESSURE DIFFERENTIAL ACROSS THE EVAPORATOR UNIT BEING DEFROSTED SUFFICIENT TO URGE REFRIGERANT CONDENSED THEREIN INTO SAID LIQUID REFIGERANT SUPPLY LINE FOR PASSAGE THROUGH THE EVAPORATOR UNITS NOT BEING DEFROSTED. 